By Brad Buecker, Contributing Editor
Many pieces of power plant equipment rely on lubricating oil, where steam and combustion turbines represent the pinnacle of these requirements. Clean, dry oil is essential to maintain healthy equipment life and prevent shutdowns that could cost hundreds of thousands of dollars.
Among others, lubricants provide the following vital services to moving machinery1:
For many applications, the typical lubricant is a mineral oil that has been highly refined. The oil base primarily consists of paraffinic hydrocarbons; for example, alkanes, with some napthenes (cycloalkanes) included.
Important properties of lube oils include:
Viscosity. Viscosity is the measurement of a fluid’s resistance to shear. It is the most important factor when selecting a lubricant for the particular application. Oil that has too low a viscosity does not provide a film of sufficient depth between surfaces. Oil that is too viscous imparts excessive fluid resistance. In most cases, the viscosity must remain relatively constant over a temperature range. Excessive thinning at high temperatures or excessive thickness at low temperatures could be problematic. The common measurement of lube oil viscosity is the centistoke (cSt). The viscosities of turbine lube oils may range from single digits at 100 C to 40 cSt or more at 40 C , whereby water has a viscosity of 1cSt and honey has a viscosity of around 1,000 cSt.”2 In fact, the code numbers for an International Organization for Standardization list of lube oils is based on the viscosity at 40 C, as outlined in Table 1.
Viscosity Index (VI). The viscosity index is a measurement of a lubricant’s viscosity stability. A low index means that the viscosity will vary widely with temperature, while a high index indicates a small viscosity change with temperature. Viscosity index can be classified as follows: low, VI below 35; medium, VI 35 to 80; high, VI 80 to 110; very high, VI above 110. 4
Pour Point. The pour point is the lowest temperature at which the oil will continue to flow. This aspect is, of course, particularly important where the oil is subjected to cold ambient air temperatures. For oils that are perhaps not highly refined, temperature reduction may cause wax (long-chain alkane) precipitation, which in turn can potentially clog small orifices and gaps between metal surfaces.
Lubricating Oil Grades
Lube oils are typically classified into five categories, as outlined in Table 2.
The common choice for many industrial lubricants today is Group II. Oils in this group have a low sulfur content (sulfur concentration of the finished lubricant may be higher due to the inclusion of specialty additives), have a large proportion of saturated hydrocarbons (alkanes) and have a high viscosity index.
Small amounts of additives are blended with lube oils, where the additive selection is based upon the application. The list below outlines several important additives and their functions.
Oxidation Inhibitors: Water, air and catalytic particulates such as iron and copper will oxidize lube oil hydrocarbons and convert them to compounds that have decreased lubricity, increased corrosive tendencies and increased varnish formation potential. Oxidation inhibitors are sacrificial chemicals that preferentially react with oxidizers. Common compounds include hindered phenols, zinc dithiophosphates, aromatic amines and alkyl sulfides. 1
Rust & Corrosion Inhibitors: Rust and corrosion inhibitors are molecules that attach to metal surfaces to provide a barrier for protection against compounds that would attack the base metal. Rust inhibitors have a polar (charged) end that attaches to the metal while the hydrocarbon end sticks out into the lube oil. Common compounds include sulfonates, phosphates and organic acids. 1
Anti-Wear (AW) and Extreme Pressure (EP) Additives: As their names imply, these compounds help to reduce wear and metal fatigue due to pressure. Under high pressure, these chemically-active additives react with component surfaces to form soft, soap-like oxide films that offer enhanced lubricity at the boundary contact between surfaces. Common AW and EP additives include zinc dithiophosphate/dialkyl dithiophosphate (AW), tri-cresyl phosphate (AW), and sulfur-phosphorous (EP). 1
Other additives may include dispersants to sequester sludge particles and detergents to clean surfaces.
The Bad Actors
Many impurities or process conditions influence the degradation of lube oil and place stress on machinery and moving components. The list includes, particulates, temperature, water and air.
Particulates may enter a lube oil system from many sources and can be generated within the system itself. Even new oil, if stored improperly, can be a source of particulates. Excessive particulate accumulation in fluid systems can cause a number of wear issues noted in Table 3. This list includes two wear-related difficulties initiated by other mechanisms.
While these factors are problematic in their own right, abrasion and corrosion can generate particulates composed of the base metals of the equipment. These particulates are typically very hard and thus can do even more mechanical damage to equipment. Also, iron and copper will catalyze oxidation reactions of the oil/additives with air and water.
Regarding temperature, heat is a lubricant’s worst enemy. A rule of thumb is the oxidative life of oil is halved for every 10 C increase in temperature.6 Heat exchangers help to dissipate heat as quickly as possible, but even then thermal effects exacerbate the formation of oxidized products, that is, the precursors to varnish. This difficulty has become particularly prominent in combustion turbine lube oil/hydraulic fluids.
Air and water are contaminants that directly influence oxidation. Water will cause other problems including reduced lubricating film thickness, corrosion and accelerated metal fatigue, while air can cause pump cavitation, foaming and higher fluid temperatures, among others.
The life of the oiland the equipment it lubricatescan be greatly extended by removing impurities before they have time to build up to catastrophic concentrations. With regard to particulates, both in-line and side-stream filtration have been used. This discussion considers just side-stream treatment, where typically a kidney loop is installed on the lube oil reservoir.
Particulates are removed by direct filtration of the side stream. Important is to select filters that will collect particles of the critical size and above for the application. Important also is selection of filters with an absolute rating vs. nominal, as the latter may allow many unwanted particles to pass through untouched.
The criteria for determining the effectiveness of a filter is the beta (β) ratio, defined as the ratio of the number of particles greater than given size [in microns] in a given volume of influent fluid to the number of particles greater than the same size in the same volume of effluent fluid. 7 For example, a filter rating of β5 =200 says that only one particle of five microns or larger will escape the filter for every 200 particles captured.
The two most common media construction materials for these filters are cellulose and glass fiber. The use of cellulose media has declined rapidly in recent years and is typically not used for high purity applications due to the inability to maintain consistent pore structure necessary for absolute performance efficiency ratings. Further, cellulose construction lacks relative strength and integrity under differential pressures and has a higher propensity for swelling, deformation and possible media migration during operation.
Water may enter a lube oil system through leaking steam seals, heat exchanger tube failures, condensation in the main lube oil tank, or other sources. Water reduces the lubricating properties of the oil, exacerbates oxidation of the oil and its protective additives, and causes corrosion and microbiological fouling in the main lube oil tank and other locations, where the corrosion products will then travel to turbine bearings and control valves, piping and so on.
Past equipment that has been used to remove water includes gravity precipitation systems with settling chambers, and centrifuges, which as the name implies, use circular motion to separate oil and water due to the difference in density. Another technology is coalescence, where a specially designed media gathers free water into large droplets that are taken from the system. Coalescers do an excellent job at removing free water, but lube oils also contain dissolved water whose quantity is very dependent upon temperature.
Thus, in locations within the system where the oil is warm, the fluid may perform at or close to specification, but as it moves to cooler locations, for instance after the lube oil cooler and in the lube oil storage tank, free water will form to cause corrosion and microbiological fouling.
A process that is capable of removing free water and up to 80 or 90 percent of dissolved water is mass transfer vacuum dehydration. The process employs mild heating, if needed, of the oil slipstream followed by vacuum-induced air flow through a small, skid-mounted unit, where the oil flows counter-currently to the air in a chamber filled with media to enhance air-liquid contact.
A turbine lube oil mass transfer/vacuum dehydration skid includes an oil heater, vacuum tower and particulate filter. Expanding air in the vacuum tower greatly lowers the humidity of the inlet air, which in turn causes the water in the oil to evaporate into the air stream. Of course, being a kidney loop application, the process takes time to remove water, but with steady operation will remove perhaps 10 to 15 gallons per day of the water from the lube oil. Unless excessive water influx is present, for example from a large leak in the lube oil cooler, the process can keep the lube oil quite dry.
Varnish formation in oil is a subject of great importance at both conventional steam plants and those with combustion turbines. Varnish occurs when oil and its additives oxidize and polymerize due to stresses placed on the fluid, which include heat transfer from process equipment, microdieseling, and electrostatic energy transfer from particulate filters.
Varnish polymers can reach high molecular weights, and due to their oxidized nature, will settle on internal components, including servo valves. The latter has become a very troublesome issue in many combustion turbines.
Power Engineering magazine reported on this issue in February 20088 with an article that outlined many of the fundamental varnish removal technologies. One of the most popular to date is a variation on electrostatic precipitation (ESP), where the device induces a charge on varnish particles for collection on oppositely charged surfaces. However, not all utility personnel have been happy with the consistency of this technology.
A process that has been recognized for some time but is again beginning to grab headlines is that of adsorption to remove varnish. While varnish is only slightly soluble in oil, the fact that it has even some solubility allows it to be continuously removed from in-service fluids without the expense and headaches of periodic off-line cleaning. Adsorption had been utilized in the past, but with mixed results. However, improved technology is leading to better performance.
Adsorption is a film-forming mechanism, where the compound to be removed exhibits an electro-chemical affinity for the surface of the collecting media. The varnish removal vessel contains multi-layer media, whose surface has been prepared to be especially attractive to oxidized varnish particles. As varnish comes out on the media, deposits within the lube oil system gradually dissolve and are subsequently removed.
Results from adsorption devices or other technologies can be tracked via the QSA (Quantitative Spectrophotometer Analysis) test offered by Analysts Inc. The procedure involves filtration of oil samples on a special media that collects dissolved varnish to produce a distinct color. The color intensity can be directly related to varnish potential indicated in Table 4.
A well designed and functioning varnish removal system should reduce the VPR to well below the “normal” value of 35. Even then, varnish has been known to accumulate in systems where VPR readings are low, so careful monitoring is very important.
1. Troyer, D., and J. Fitch, Oil Analysis Basics, Noria Corp., Tulsa, Okla., 1999.
2. M. Johnson, “ISO Viscosity Grades”; Machinery Lubrication, July 2001.
4. Web site outline of Lubricating Oils and Additives, a course offering by PDHengineer.com, Houston, Texas.
5. Kramer, D.C., Lok, B.K., Kurg, R.R., and J.M. Rosenbaum, “The Evolution of Base Oil Technology Industry Focus”; Lube-Tips (date unknown), The Noria Learning Center, www.noria.com.
6. D.D. Troyer, Noria Corp., and S. Gabarin, “Lubricant Lifecycle Management Part 1”; Machinery Lubrication, March 2007.
7. Contamination Control and Filtration Fundamentals, technical brochure from the Pall Corp.
8. F. Guerzoni, “Eliminating Varnish”, Power Engineering, February 2009.
Author: Brad Buecker is Technical Support Specialist for AEC PowerFlow, Kansas City, MO.
Lubrication Helps Extend Run-times
By Gene Finner, STLE Certified Lubrication Specialist, Dow Corning
Throughout the power generation industry, a major trend is to improve efficiency by extending the intervals between outages. One way to decrease outages is through proper maintenance and lubrication of power generation equipment. Today, many power plant operators are revisiting their choice of lubricant to improve reliability, extend maintenance periods and reduce outages.
Proper lubricant choice is often overlooked in favor of more complex and costly maintenance programs. However, many synthetic lubricants available today can help decrease outages and increase the intervals between scheduled maintenance.
Synthetic materials can be formulated into many different lubricant forms such as greases, pastes, compounds, dispersions and dry film bonded coatings. Depending on the machine part being lubricated, the conditions present and the application case desired, different lubricant forms may perform more effectively than others.
Greases are used widely because they are long-lasting and easy to use. Greases are simply oils in thickeners with additives and corrosion inhibitors. The thickener releases minute amounts of oil over time for long-term lubrication, extending equipment life and reducing the time between outages. Specialty greases such as fluorosilicones can provide extended resistance to temperature extremes and washout by water or other chemicals.
Fluorosilicone grease is particularly effective for circuit-breaker bearings because of its wide temperature capability, resistance to washout by penetrating oils, heavy load carrying capability and resistance to physical change, such as drying out, over time. Fluorosilicone grease can help significantly extend service life when compared to general purpose type products.
Lubricating pastes are solid lubricants mixed in oil, which are used on threaded connections, pins and bushings, and for assembly, press-fits and run-in. They are also effective on sliding surfaces, guides, ways and acme screws. An anti-seize paste that does not contain molybdenum disulphide is recommended for the threaded connections on circuit-breakers. Pastes may offer an advantage in prevention of galling and seizure and often exhibit excellent long term corrosion protection. This feature is particularly useful for reducing or eliminating seizure of infrequently moving parts.
Silicone compounds are made of silicone fluid thickened with amorphous silica. They are naturally waterproof and lubricate rubber very well. They are electrical insulators and effective release agents, and are often used to protect and rejuvenate rubber. On sub-station circuit-breakers, silicone compounds are recommended to lubricate the o-rings on pilot control valves, protect door gaskets, and act as a dressing for compressor drive belts.
Dry film bonded coatings are formulations of solid lubricants and curing resins dispersed in a solvent. When applied to a surface, the solvent evaporates leaving a curing resin filled with solid lubricants that cure to form a dry lubricant coating. These coatings provide lasting, heavy-duty lubrication for a variety of surfaces. They will not collect dust or dirt and can be used in dirty environments such as fossil fuel-driven power plants. They have very good resistance to oils and solvents, including penetrating oils.
Dry film bonded lubricants excel in low speed sliding applications in difficult environments. They are recommended for use on circuit-breakers during overhaul and can be applied to cams, slides, ways, guides, linkage pins and flats, and acme screws.
Penetrating oils are usually made of petroleum oils with or without solid lubricants such as molybdenum disulphide, graphite and PTFE in a solvent carrier. While high-quality penetrating oils can be effective for busting rust and temporary lubrication at points of friction, power generation plant operators should be very careful about which penetrating oils are used for any kind of lubrication. These products tend to act as solvents and can wash out most grease and oil.
Low-quality penetrating oils may leave sticky varnish-like residues where applied. Be careful to avoid drenching with penetrating oils as some can wash greases from bearings. In fact, frequent drenching with poor quality penetrating oils may be worse than no interim lubrication at all. Plant operators should be sure to use high-quality penetrating oil and carefully select specific applications on which to use high-quality penetrating oil to reduce maintenance and extend the time between outages.
Silicone dispersions are usually made by dispersing solid lubricants in solvent. They are used as release agents and lubricants for rubber, plastic, metal and any of their combinations except metal to metal. These dispersions can be used for many of the same applications that the silicone compounds are applied. They simply provide an easier form of delivery.
Along with use of today’s modern synthetic lubricants, knowledge of how they work, how to apply them and how much to apply can make a difference in performance. Application techniques are different for pastes and greases; plant operators should be sure to understand the strengths and weaknesses of different product types. Quality lubricant suppliers such as Dow Corning are ready to provide maintenance personnel with technical training and knowledge that can lead to increased equipment reliability.